Please see the file /etc/apt/sources.list
Peruse the Debian method of marking [oldoldstable] [oldstable] [stable] [testing] [unstable] [experimental] and various other kinds.
Then find your installed Knoppix version's (not the Debian Current Version) declaration in the sources file at the very top.
Re-arrange your declarations accordingly. It is simple and common-sense.
Also don't forget to install the security keys by hand.
Can't be done unless you are Superuser.
That is all there is.
WD 2TB Certified Refurbished Elements, External Hard Drive - RWDBU6Y0020BBK-WESN
$49.99
Seagate Exos X16 ST14000NM001G 14TB 512E SATA 6Gb/s 3.5" Enterprise Hard Drive
$157.99
Seagate Exos X22 ST20000NM004E 20TB 512E SATA 6Gb/s 3.5" Enterprise Hard Drive
$268.99
Western Digital 4TB WD Purple Surveillance HDD, Internal Hard Drive - WD43PURZ
$96.99
HGST Ultrastar DC HC520 12TB SATA 6Gb 256MB 3.5" Enterprise HDD- HUH721212ALE601
$82.99
Seagate Exos 7E10 ST2000NM000B 2TB 7200RPM SATA 6.0Gb/s 3.5" Internal Hard Drive
$29.99
HGST Ultrastar HE10 HUH721010ALE600 10TB SATA 6Gb/s 7200RPM 3.5" Enterprise HDD
$69.99
8TB Seagate Archive SATA 3.5" HDD Hard Drive 100% Healthy 200MB/s ST8000AS0002
$36.57
WD 16TB Elements Desktop, Certified Refurbished Hard Drive - RWDBWLG0160HBK-NESN
$174.99
WD 4TB Certified Refurbished Elements, External Hard Drive - RWDBU6Y0040BBK-WESN
$69.99